From One Battery to Another
July 15, 2013 6:45 PM   Subscribe

How long can I use the battery in my vehicle to charge my smartphone without the vehicle running before I drain the battery too much to subsequently start my vehicle?

I have been accepted as a vendor to sell my stuff at an outdoor festival! But, my items are expensive, so I use a credit card app on my smartphone so people can buy my things. In fact, I was told that all the vendors use smartphone apps. This is great.

But...there is no electricity available for four long days...and no electricity overnight, either.

I asked the event person how the vendors all use smartphone apps without electricity, and got no response.

I bought a crank charger, but it could take several hours of continuous cranking to keep my phone charged. My items will be popular and I will be using my smartphone constantly for the four days.

My 2008 Dodge Grand Caravan has two outlets I can use while the van is not running. I can use this to charge my phone, but how long can I do this before I drain the van's battery too much and ruin it or not be able to start my van? I cannot have my van running at the same time since it will be located within a venue with lots of people, etc.

Anyone know how I can calculate this? Or...maybe there are other charging ideas, as well.
posted by TinWhistle to Travel & Transportation (16 answers total) 3 users marked this as a favorite
You can get portable battery chargers, solar battery chargers, or get a few additional batteries and charge them before you get to the festival (unless you have an iphone or other phone without a removable battery.) You can charge the extra batteries in your cell phone before you go or you can get a $10 external battery charger.
posted by needs more cowbell at 6:59 PM on July 15, 2013 [1 favorite]

Anker Astro Pro claims to provide 7 charge cycles to a phone and the Pro2 claims 10. I have not used these products but I have had a good experience with the other Anker items I use constantly (non-OEM cell phone battery & external battery charger).
posted by needs more cowbell at 7:03 PM on July 15, 2013

Even if you have an iPhone, there are all kinds of chargers, some of which run on regular batteries, at a variety of price points. We picked up a couple at CVS for something akin to $15 each right after Hurricane Sandy. (They may not be good for more than one complete charge per set of batteries, so be prepared to go through a TON of batteries.)
posted by Andrhia at 7:04 PM on July 15, 2013

From dark experience, I can tell you that a twelve-year-old VW with a two-year-old battery lasted about 40 minutes before it needed to be jump started. And that was just when I was charging a phone, nothing else. I would definitely go with a different charging plan, and probably would get a backup phone battery, so one can charge while you're using the other.
posted by corey flood at 7:05 PM on July 15, 2013

It really depends on the battery you have installed and its newness, size, etc, but considering most movies are about two hours and I've been to drive ins where you use your car speakers, (as well as everyone else, Dodge Caravans included). I'd say two hours is very safe for a newer battery. After this point, if you still need it without running your car for a bit, you can keep a general eye on your battery and test it by turning on the parking lights with the door open. If the dome light dims significantly, your battery is getting weak. You shouldn't continue to do this as you can ruin the battery. Worst case scenario you can bring along a portable car battery charger. Handy to have in any case, even if you don't end up using it.

With that said, If its a large festival, I'm sure generators will abound. Buy a small residential one from a home improvement store, or failing that you'll likely be able to piggy back quick charge ups from a neighbor, perhaps for a small token of appreciation.

I'd speak with an event organizer to get the ins and outs of this. Unless it's Eco-themed, there's usually a bunch of generators for food and beverages at the very least.

Low wattage home use ones are far quieter than the beefier commercial grade units if you're afraid about noise being an issue.
posted by Debaser626 at 7:07 PM on July 15, 2013

I can speak to the Anker batteries, though I'm not sure any of them will last the entire time for you. Maybe, if you only need to recharge twice. I'd get a Goal Zero solar kit because they're useful for a lot of other activities and about the same price.
posted by kcm at 7:16 PM on July 15, 2013

In a perfect world, this should work fine. The battery has plenty of capacity. However, if you are charging it up with the keys on, lots of other things are running down the battery. So the best bet would be to charge it with a car charger plugged into an always on cigarette lighter adapter. That way the only drain on the battery is something like a maximum of 1.5 amp hours. Using the onboard inverter is bound to drain the battery at a much higher rate.

I, personally, would find some other way out. Or at least have some kind of backup plan. (Like the alkaline battery topper upper.)

Given that this is a profit making opportunity, I personally might even consider buying another car battery to have in reserve. That way if you drain the installed battery (whose life is 1/4 to 1/2 over anyway), you have a spare ready to go and you will be able to gauge how much strain charging your phone puts on the battery.
posted by gjc at 7:17 PM on July 15, 2013

I would recommend a solar charger for this, you can get one for a car (and use your car outlets) or get one specifically for charging a cell phone.

The car battery probably has 100's of cell phone battery charges in it however by turning on your vehicle you are energizing the whole electrical system and that is a much, much bigger load that will drain the battery down in a few hours, or pretty much what gjc says on preview.

Another factor is that typical lead acid batteries (like in cars) do NOT like having a small long term load on them that slowly draws them down, After such a load is applied and the charge in the battery drops to less than 50% you start damaging the battery, and the lower you go the more you damage the battery. It takes a few times of this to really harm a lead acid battery but it does happen (BTW deep cycle batteries can do this without harm but they are not really suitable for car charging sytems usually).
posted by bartonlong at 7:22 PM on July 15, 2013 [1 favorite]

If it really is for four days (you can't charge at night? You're camping?) then you can pick up a generator for about $150 or so. I'm assuming you'd have other electrical needs at night. If you have a marine or truck battery wired to a phone charger you could easily get through the time charging the battery every 5 or 6 hours during the day and it'd last you over night easily.

Alternatively, a solar charger that can charge external battery extenders (maybe one on charge and one on the phone?) and rotate them as you go.
posted by Brockles at 7:30 PM on July 15, 2013

Yeah, there are all kinds of chargers, MUCH cheaper then having to replace your car battery (or having your car towed because it won't start!). Maybe have your phone behind your booth or table outside and keep it in a solar charger when you aren't using it during the day. Charge it up overnight if they have generators you can use to do that.

I'd also turn off all the other apps on your phone that eat up battery space that you can possibly do without for a few days. If even get service there then put your email to manual updates, disable game notifications, turn off updates, turn off texting, etc.
posted by misha at 7:38 PM on July 15, 2013

When we do (did? darned baby!) art festivals we don't have the luxury of being next to our vehicle. So my solution for this was to buy a deep-cycle battery and use it to power an inverter, though we had greater needs - including for demoing merch - than you're describing.

The issue with using a car battery - in your vehicle or carting one around - is that car batteries are really not meant for sustained discharge. They're designed primarily to put out a big chunk of power in a short time to crank your starter. You probably will be able to use it for a low-impact use like charging a single mobile device without issue, though this sort of thing shortens the life of the battery if you're repeatedly discharging it fully (which you certainly hope you are not).

Your question indicates you don't have to have the car on to use those outlets. I assume by outlets you mean cigarette lighter plugs; if the van has an A/C inverter in it that's a higher consumption for this purpose and you're way more likely to run it down.

If you stick with this plan I'd say you're probably fine through the day and can simply start the car in the evening and let it idle for 15-20 minutes. That's likely enough time for the alternator to top off the battery if you don't turn any lights or other systems on, given your described use.

HOWEVER, I think a better solution would be for you to just get an external battery for specifically this purpose. I bought one of these earlier in the year and it'll fully recharge an iphone 5 for me at least 4 times before recharge; I've never tried taking it longer.

If you want to provide more juice I'd get a deep-cycle battery and a good inverter and perhaps a solar panel. That's the setup I use at the show and we run a laptop and several devices as well as demoing lights and other things on it. The battery and inverter are about $110 at a Walmart type store and panels vary greatly. You'll also want a trickle charger to recharge it via home current and maybe a small folding cart to transport it - they're heavy!
posted by phearlez at 7:40 PM on July 15, 2013 [1 favorite]

I posted this in an earlier thread, but you are going down the right route with the car battery idea. Just not the main one you use in your car.

A roll-your-own solution here will be much cheaper than anything else.

First of all, go on google maps and yelp and look for "auto battery" shops in the crappy/industrial district part of your town. The one in mine sells reconditioned cop car batteries for $30. These are larger than most normal non-truck car batteries and also come with a guarantee and come charged if you ask for it.

Now go on amazon and buy this, this, and a decent little charger. Total outlay here should be about $60-70. And compared to all the crappy "car jumper boxes" that are in the $60-100 price range or even above this will annihilate then. You could run a laptop off of one of these larger car batteries for days.

This will run your phone for weeks i'd bet. And never run a phone(or laptop, or anything you don't have to) off of an inverter if you can help it. That's what the outlets in your car would be, and converting the power from 12v>120v then 120v>5v for your phone will just be losing tons of energy where you don't need to. A little 12v>5v converter loses almost nothing and won't drag on the battery much of at all. Either will the phone.

I've recommended this setup to people doing booths at conventions, stuff in the woods like you, and who just wanted to run laptops/other electronics in non-running vehicles.

I've used a larger version of this setup(with a decent inverter) to run an entire RV full of gear for 24 hours. Totally kicked ass.

Fuck generators. I'd scale up something like this before i'd start burning gas. You can re-use this setup for years if you take care of the batteries for nothing but the cheap initial outlay.

as a side note, jesus christ, amazon has the worst search in the world
posted by emptythought at 8:31 PM on July 15, 2013 [12 favorites]

I looked up some numbers. Smartphone batteries store a charge 1000-2000 mAH (milliamp-hours) range, while car batteries are more like 45 amp-hours, or 45,000 mAH, if you will. Car batteries are rated differently, using a metric called Cold Cranking Amperage, because it has to provide a much higher current/amperage for a short time during start (and in cold weather) in addition to providing a nominal current during normal operation, while your phone battery's draw is has a more narrow range for the battery to provide.

It's not safe to assume that charging from this battery will get you anywhere near 45000 mAH because of losses in the transfer, but it yes, you can charge your phone (plus some standby batteries) without worry over 4 days. To be on the safe side, I would fire up the car after the second day and idle it for 30-60 minutes (or take a drive); that'll recharge the battery.

The problem you're likely to encounter, assuming a brisk business, is running out midday, so you'll want to be hooked into some kind of charger at all times; I'd recommend a largish (3000-5000mAH or greater) battery you can hook directly too. Also, add some kind of attachment that you can lock down; what makes a phone good to carry also makes it easy to swipe.
posted by Sunburnt at 9:32 PM on July 15, 2013 [1 favorite]

Number of recharges = Usable energy in car battery / energy in phone battery * efficiency

Energy = voltage * amp-hours

I would say not to take out more than 22 Ah from a Grand-Caravan-sized car battery before recharging.

# recharges = 12V * 22 Ah / ( 4V * 2 Ah ) * .75 = 25 recharges from empty (I used worse-case numbers)

This assumes you have the car off when charging, so the other stuff isn't draining it. bartonlong is correct that deep discharges will kill car batteries, but if you only do it < 10 times in 7 years, it won't be detectable.

Finally, worse case, if you draw too much just ask for a jump. No big deal.
posted by flimflam at 10:02 PM on July 15, 2013 [1 favorite]

I have both the Goal Zero Nomad 7 and 10, with the little battery pack. They both work great. You can charge both the phone and the battery pack off the solar panel at the same time, then use the battery pack to charge the phone when there is no sun. It works just fine even in indirect sunlight. Cloudy days aren't a problem.
posted by spikeleemajortomdickandharryconnickjrmints at 5:19 AM on July 16, 2013 [2 favorites]

FWIW, the large size Optima Yellow Top batteries are good for 75 amp-hours at 12V. Buy one, charge it up and plop it in the back of the van with something like this socket and a 12V charger for your phone. You'll be good for a week and if it goes dead, that's OK, it's a deep cycle battery. Just remember to charge it up when you get home; they tolerate storage better when mostly charged.
posted by wierdo at 11:45 AM on July 16, 2013

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